The lower the environment temperature is, the greater the demand for heating capacity of air conditioner is. However, at −20° C., an existing heat-pump with single-stage compression can only be started normally, but its heating capacity is severely attenuated, so the heating effect cannot be guaranteed, and the reliability of the air conditioner is also severely challenged.
A two-stage compression system with enthalpy-increase by gas secondary injection has larger heating capacity and higher energy efficiency than a heat-pump with single-stage compression at a low temperature. Compared to the single-stage compressor, the two-stage compression system can reduce a pressure ratio and temperature of discharged air, and can increase air suction efficiency and compression efficiency, thereby increasing the heating capacity and the heating efficiency.
A two-stage compression with enthalpy-increase by gas secondary injection, includes a high-pressure stage compression and a low-pressure stage compression, and has two or more cylinders in which the cylinder used for a first stage compressor is called as a low-pressure cylinder and the cylinder used for a second stage compressor is called as a high-pressure cylinder. The principle of enthalpy-increase by gas secondary injection is that a gaseous refrigerant which is injected into an air suctioned port of the high-pressure cylinder of the compressor from a port for increasing enthalpy by gas secondary injection positioned at the middle portion of the compressor is mixed up with a discharged refrigerant compressed by the low pressure cylinder, and then is compressed in the high pressure cylinder.
In a two-stage compression system with enthalpy-increase by gas secondary injection, the secondary injected gas has a very important influence on the system performance and reliability. The secondary injected gas mixed with a liquid will cause dilution of lubricant in the compressor. Due to the impact and incompressibility of the liquid, a lot of liquid refrigerant entering the compressor cylinder at a higher speed will cause air suction valve to break or bend excessively, and will cause severe wear of the cylinder. Closing the control valve of the secondary injected gas can effectively avoid the condition that liquid is carried in the secondary injected gas and will be benefit of the long-term operation of the compressor, but the performance of the two-stage compression system will be greatly reduced.
Thus, the control valve of the secondary injected gas needs to be always open and also needs to be closed in time in the case that liquid is carried in the secondary injected gas. The performance and reliability of the two-stage compression system will be affected by the accuracy of the judgment to whether or not liquid is carried in the secondary injected gas. At present, the method of detecting superheat degree of the secondary injected gas is commonly used to determine whether or not liquid is carried in the secondary injected gas. This method can only determine whether or not liquid is carried in the secondary injected gas. This method has the following deficiencies: Firstly, in the case that a temperature-sensing package for secondary injected gas is disposed after the gas secondary injection valve, the detection temperature of the secondary injected gas will be decreased due to a certain throttling action of the gas secondary injection valve, which will result in that the gas secondary injection valve is often closed because the superheat degree of the secondary injected gas is detected to be critically low; and secondly, in the case that the temperature-sensing package for secondary injected gas is disposed before the gas secondary injection valve, the detected temperature of the secondary injected gas is higher and the superheat degree of the secondary injected gas is larger, which will result in a situation that a small amount of liquid being carried in the secondary injected gas cannot be detected, thus the reliability of the compressor cannot be ensured.